Change SkDecodingImageGenerator API

src/lazy/SkDiscardablePixelRef.h

Index: src/lazy/SkDiscardablePixelRef.h
diff --git a/src/lazy/SkDiscardablePixelRef.h b/src/lazy/SkDiscardablePixelRef.h
index 0a727b6367eb80172324cf9d4aed72f60b7c07d2..72a2ed93bede91b395e5f805649f907d420a1048 100644
--- a/src/lazy/SkDiscardablePixelRef.h
+++ b/src/lazy/SkDiscardablePixelRef.h
@@ -13,6 +13,15 @@
 #include "SkImageInfo.h"
 #include "SkPixelRef.h"
 
+/**
+ *  A PixelRef backed by SkDiscardableMemory, with the ability to
+ *  re-generate the pixels (via a SkImageGenerator) if the DM is
+ *  purged.
+ *
+ *  Since SkColorTable is reference-counted, we do not support indexed
+ *  color with this class; there would be no way for the discardable
+ *  memory system to unref the color table.
+ */
 class SkDiscardablePixelRef : public SkPixelRef {
 public:
     SK_DECLARE_UNFLATTENABLE_OBJECT()